(06-09) 08:25 PDT SAN FRANCISCO -- It's going to be a scorcher Monday, with temperatures breaking into the triple digits in some areas before a cool-off starts Tuesday, forecasters said.

A high-pressure system hovering over the area since Sunday is turning up the heat, with highs in the hottest areas such as Livermore and Antioch expected to hit 100 degrees, said Steve Anderson, a forecaster for the National Weather Service.

The North and South Bay will also stay hot. Residents can expect temperatures in the mid-90s to 100.

In San Francisco, the heat will top out around 78 before late-afternoon fog rolls in, the weather service said.

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"It's going to be a sunny, warm day. It's a good day to extend the weekend if you can," Anderson said.

The high pressure will start to move southeast Tuesday, making way for a more typical onshore flow, Anderson said. The 100-degree highs should drop to the 70s around the region by Wednesday and stay that way into the weekend.

Monday's heat prompted Bay Area air-quality authorities to issue a Spare the Air alert for the second day in a row. Officials ask people to restrict their driving and use of gas-powered garden equipment.